The present invention relates to a radar system using a long pulse, and more particularly to a pulse compressing apparatus which is required when a received long pulse reflected from a target is to be converted into a radar video signal.
In such a radar system, a long pulse, whose carrier frequency is subject to linear modulation or linearly changes with a chirp signal, is used, because the long pulse is more advantageous in range resolution and accuracy and also it provides greater transmit pulse energy while applying a pulse compression operation. Where such a long pulse is used, the pulse should be compressed as disclosed, for example, in Merrill I. Skolnik, Introduction to RADAR SYSTEMS, International student edition (McGraw-Hill KOGAKUSHA, LTD., 1962), 10.9 Pulse Compression in Chapter 10.
This pulse compression can be represented by correlation processing indicated by Equation (1):
Pulse compression output y(t) ##EQU1## where x.sub.r (.tau.) is a received signal and ref (t), a reference signal. Whereas the reference signal is so determined as to maximize the compressed output, or as to result in a matched filter, the complex conjugate signal of a transmitted signal is used as the reference signal because the received signal is a reflection of the transmitted signal.
It is known to those skilled in the art that correlation processing in the time domain can be represented by a multiplication in the frequency domain, so that the Equation (1) can be represented by Equation (2) below in the frequency domain: EQU Y(f)=X.sub.r (f).multidot.X.sup.*.sub.t (f) (2)
where Y(f) is the Fourier transform of the compressed output y(t); X.sub.r (f), that of the received signal x.sub.r (t); X.sup.*.sub.t (f), that of the complex conjugate signal of the transmitted signal (i.e. the reference signal ref(t))
A digital pulse compressing apparatus according to the prior art is a materialization of Equation (2), and may have such a configuration as illustrated in FIG. 1, for instance. Referring to FIG. 1, a received signal and a transmitted signal (which is a complex conjugate signal) are respectively sampled and converted into digital signals by analog-to-digital (A/D) converters 201a and 201b and, after effects of the sampling are reduced by window function processors 202a and 202b, undergo a Fourier transformation at high speed by fast Fourier transform (FFT) processors 203a and 203b. The outputs of the FET processors 203a and 203b are multiplied by a mulTiplier 204, and the multiplied output undergoes inverse FFT processing by an inverse FFT processor 205 to give a pulse compressed signal in the time domain.
Since a radar system which, because of its task to scan space, requires pulse compression on a real time basis, pipeline FFT processors as the FFT processors 203a and 203b are often utilized. Such pipeline FFT processors are described in detail, for instance, in the U.S. Pat. No. 4,222,050, "Moving Target Indication Radar".
In a radar system, conditions to the distance to the target to be tracked and the state of clutter are not constant but vary, so that the system should be adaptively responsive to such variations. A radar system using a long pulse can meet this requirement by adaptively altering the pulse compression ratio. As well known, the pulse compression ratio is defined by B.sub.96 where .tau. is a duration of the long pulse and B=f.sub.2 -f.sub.1, a carrier frequency change from f.sub.1 to f.sub.2 over the duration. Thus, in order to alter the pulse compression ratio, the number of sampled data points to be processed in digital pulse compression processing, corresponding to the duration of the long pulse, is altered.
In the prior art digital pulse compressing apparatus described above which uses FFT processing, however, the number of sampled data points to be processed is fixed, and it is difficult to alter this number. In the pipeline FFT processor for real time continuous FFT processing in particular, in order to change the number of data points to be processed the extent of delay by a delay means and the number of processing stages should be changed, entailing an extremely complex configuration. There would further be required window function processing to suppress the effects of sampling and an inverse FFT processor to return the signal domain from the frequency domain to the time domain A digital pulse compressing apparatus using conventional FFT processing therefore involves the problem that it is difficult to adaptively alter the pulse compression ratio while achieving real time continuous processing.